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How did the 767/757 reduce fuel burn relative to older aircraft (like the 727)?
When Boeing launched the 757 and 767 aircraft, they widely touted it's improved fuel efficiency relative to previous airliners.  Notably:

Delta stated that the 767 was "quieter and 30 percent more fuel-efficient than Delta's older jets it replaced." Source
Guy Norris stated, "aerodynamic improvements contributed to the 30% fuel burn savings that were claimed for the 767 over previous types" Source

In what ways did Boeing engineer's improve the design of the aircraft to lower the fuel burn of the aircraft? Of course, propulsion advances (e.g. GE's CF6 engine) played a huge role. However, my interest is in other aspects of the design, most specifically the design of the wing and other aerostructures.

The primary driver of reduced fuel burn through the history of gas turbine-powered flight is increased efficiency of the engines, which comes from two things: 1) new blade materials that permit operation at higher temperatures and 2) bypassing the core and generating thrust with a fan.

Definitely understand that! And I picked up a couple of great books on the evolution of engines. For example: https://www.amazon.com/Starting-Something-Big-Commercial-Emergence/dp/1563472899. That's why I asked about wings & aerodynamics specifically, since that's what I know the least about :)

Your Norris source states most of the aero improvements came from the wing.

The airfoil mentioned has a supercritical section. Below is a typical supercritical section compared with one from the 707. The actual 767 section is Boeing proprietary and its profile is not likely to be easily found. Shown is the comparatively greater thickness and aft camber of the supercritical airfoil.

Important for fuel burn reduction is the drag characteristic these sections give to the wing. Comparing the drag polars for the two sections you see that the minimum drag is similar, but that the 'drag bucket' for the supercritical section is four or five times wider than that of the 707.
This matters because aircraft have to operate at different weights due to payload, flight length and fuel burning off during a flight. An aircraft with the drag characteristic of the supercritical section can stay in the low drag region longer, while the 707 section equipped aircraft has to move up the curve into higher drag and thus higher fuel burn. The angle (alpha) at which minimum occurs doesn't matter as aircraft are designed to take this into account.

Thanks for the answer! That raises another question in my mind, which is whether the supercritical aerofoil was improved over time for future aircraft, like the 777. That is, the goal of a supercritical aerofoil, as I understand it, is to delay the onset of the shock and flatten the pressure distribution over the top wing. Did post-767 aircraft do that "better" than the 767 did in some way?


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Can a gas turbine be able to switch from torque to thrust?
Can a turbine engine be switched from providing torque (like a helicopter) to the rotor to transfer power to provide exhaust thrust (like a jet plane)?
Related: BV-347 upgrade this way?

I suppose the F135-PW-400 powerplant on the F-35B does just that.
A turbine engine is used to create high enthalpy gas which can either pass through a turbine to produce mechanical work or pass through a diffuser to accelerate it and create a reaction impulse.  General Electric has designed power generation gas turbines which use the gas core from the GE F404 fighter engine changing from a thrust application to a power application.
As to doing this on the fly, other than the F-35B application I don’t know of a gas turbine which can dynamically change between the two.  High bypass turbofans do get a small portion of their total thrust from a hot diffuser for turbine exhaust gases as do turbopropeller engines.  But a total dynamic shift from one to the other?  No I’m not aware of that.

This isn't the same as converting a turbine connected mechanically to some system to a thrust turbine.

The main way to convert a gas turbine to provide thrust rather than shaft horsepower is to attach a large fan or propeller in place of the previous load. For example, the GE CF6, which powers aircraft such as the 747, 767, and A330, has a gas turbine variant called the LM6000 that provides around 50 MW of power.
If you want to provide thrust from the exhaust, you're talking about a turbojet, which will require a lot of changes. A gas turbine like that in the CF6 is designed to extract most of the power out of the exhaust gas to drive a load such as a fan or a generator. Just removing that load is going to cause the engine to be unstable. Each stage in a turbine engine is dependent on the stages around it for smooth operation and even small changes can cause problems. You might be able to salvage the high speed spool but the rest will have to be redesigned to provide a stable compressor section and provide most of the thrust from exhaust.
Turbojets are very loud and only start to be an efficient choice at high speeds. Even modern fighter jets are powered by low-bypass turbofans.

Most turbine engines designed to drive some sort of mechanical system such as a hydraulic pump and electrical generators are not designed to provide thrust because their fuel controller can only run it at one speed. In other words, there is no throttle. Also, the internal components may not be designed for that type of application. So while it is in theory possible, it can be expensive and can result in some interesting failure modes that will entertain people standing far away.

Those standing close by will probably not be quite so entertained by any of the failure modes possible.

@FreeMan What does this have to do with my answer? I said nothing about people standing close by.

"result in some interesting failure modes that will entertain people standing far away", but may not be quite as entertaining to those standing closer. #HumorFail

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Would this modified tire work to prespin an aircraft tire?
In order to decrease that initial hit of the tire spinning up to ground speed on the runway, could adding a blades or shape into the tire rim of a commercial plane save on tires by prespinning the tire like a pin-wheel prior to landing?
Related: Why are aircraft tires not pre-spun prior to landing to preserve them?

What problem are you trying to solve?

Cheaper just to recap the tires over and over until the carcass is shot.

The idea is good in concept but it would be pretty difficult to match the rpms to the actuual ground speed, in fact if the wheel was spinning faster it would actually wear the tyre more!
Another factor is that aircraft wheels often do not hit the runway straight on. In a crosswind situation they could be off a few degrees so the tyre would still scrub.
Also the fact that adding weight is anathema to airplane designers, not only the motors but electronic controllers and associated wiring. This weight will have to be carried for the duration of the flight.
Finally the hardest working part is not the tyre but the wheel bearings.. on a longer widebody flight they go from cold-soaked to very hot in a few seconds. Tyres actually fare pretty well, I've seen new tyres fitted and come back after a few sectors which still looked pretty new.
As mentioned by John K in the comments above, its cheaper to recap they tyre.

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What is the fastest unpressurized aircraft?
What is the fastest unpressurized aircraft with oxygen  masks that can carry people?

Living people or dead (dying) ones?

What was the highest start for a skydive not involving a pressure suit and how fast did the skydiver get going?  Or did you mean in level flight?   Also see other comment - consider any WW2 fighter, nearly all of which were unpressurized.  With a mask you can go way above 16K without pressurization!

I'd say X-15. Don't think it was pressurized and it can go mach 6.

Also, you could just sit in the top of an ICBM and bring your own oxygen. That will carry you. You probably won't make it, but you would be going at a good rate for sure

@Pheric all of the X planes were in some capacity pressurized going back to the X-1 and including the X-15. The all mainly used a Nitrogen pressurization system with pilot fed oxygen to effectively keep the cockpits fireproof.

I wonder if any of the early jets were unpressurized?  Especially F-86 generation or later?

Being petty here perhaps, but aircraft capable of being pressurized are also capable of dumping cabin pressure.  Therefore an F-18 Hornet could be unpressurized and still go mach 1.  So what you are really asking is "what is the fastest aircraft that is incapable of being pressurized?"

X-15 didn't carry people tho, it only carried a person.  SR71 is probably the fastest more-than-1 seater, altho both would been considered crew.  If "people" are taken as other those other than required crew, that make the question different.

Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor.

The operational ceiling was limited by what the pilot could endure for
  several minutes while breathing oxygen from a mask, without losing
  consciousness. Pilots underwent altitude chamber training to harden
  them.

Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.
The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h.
About 370 were built.

There's a repro ME163 in flying condition (at least as of a few years ago, when I read about it).  Said to require care to avoid transonic effects.

In the height of 16000 feet, a human can survive on normal atmospheric pressure. Above 16000 oxygen level keep getting down and changes on pressure.
So we are talking about the aircraft which flies till 16000 feet. Cirrus SR22, Beechcraft Bonanza, Cessna 172, Cessna 182 Skylane, Cessna 310 are some fastest unpressurized aircraft.

Er... Even DC-3 (which is still flying) is faster than any of these, and it is surely unpressurised. There are/were plenty of much faster ones. Nearly any airplane of WW2, for example.

Yes, absolutely right.

Also, the question asked "with oxygen masks". I don't think any of the aircraft you mentioned have oxygen masks.

@Bianfable A family's friend had a Cessna TR-182 some 20 years ago and he used an oxygen mask when he flew without passengers to be able to fligh at higher altitudes. But he also had extra fuel tanks allowing him to stay airborne for 9 hours, so I don't consider it a normal configuration.

Cirrus SR22 ... are some fastest unpressurized aircraft citation needed

A Lancair will blow the doors off of a Cirrus.

But that would depressurize it and invalidate the comparison

@zymhan A Cirrus pilot might wish she had a Citation instead, but the latter is pressurized


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Could a helicopter fly like a plane this way?
Could a wing be added to a helicopter to allow the entire craft to tip 90 degrees forward allowing it to fly faster like a plane using a fixed wing and leaning into the wing for lift?
I understand that there would be major modifications to be made but would there be any benefit?

It looks very draggy, best to tilt the body 90° CCW and call it an aeroplane.

@Koyovis Yes,I know. A Cobra helicopter would be more aerodynamic and the overall shape would have to change to be streamline at that angle.

Is a V-22 Osprey what you’re looking for?

Another helicopter that could be considered a hybrid is the Eurocopter X³ (x-cubed) which has short wings with a propeller attached on each side and no tail rotor. It flies some 255kts (472 km/h) according to wikipedia: https://en.wikipedia.org/wiki/Eurocopter_X%C2%B3

The V22 Osprey is essentially a twin rotor helicopter that can reposition its engines and propellers to fly like a plane.

Also see tiltwing aircraft.

Google "blackfly airplane"

@quietflyer will do also added answer.

@quietflyer yes seen its supposed to be available next year at $80k

You've described a tail-sitter aircraft. Instead of a helicopter that tilts sideways to cruise, it's a airplane that can sit on its tail and uses thrust to take off vertically, before transitioning to fixed-wing horizontal flight. This makes more sense since a plane spends more time in cruise than takeoff.
They were first demonstrated by Nazi Germany during WWII. A major benefit over the tiltrotor or tiltwing is that you don't need the bearings that can support the thrust of the engines or lift of the wings. You can also use a single centerline engine. Existing designs use contra-rotating propellers or jet thrust vectoring for anti-torque instead of the helicopter tail rotor.
The idea has been revived recently for UAVs, which don't have the occupant comfort problem:

Aerovironment SkyTote (US Government photo)

What about a wing setter?

There would certainly be a great benefit in occupant comfort!
The aircraft would get greatly increased drag of its large frontal cross-section and lose most of the lift that the rotary wing normally produces. It's not just a propeller turned upwards, but an actual wing, producing lift on the same principles. Helicopters require far less power to fly forward than to hover thanks to this dynamic lift.
Some occupants don't appreciate the improved ergonomics of the depicted position and prefer that only the rotor is tilted forward. To them, the solution is tiltrotors like the V-22 - you get to use the rotors as huge propellers, but not the massive drag from tilting the entire plane. Note the rotors are also sized somewhat smaller than for a pure helicopter, and still their size is a source of extra drag in flight.
In the drone world, when designing from scratch, there's been a much more elegant solution - the Parrot Swing. And yes, it's probably the first aircraft to look like the X-wing for entirely practical reasons. But remember that quadcopters (drones) are not normal helicopters; they use differential thrust from simple fixed propellers for control, not complicated rotor assemblies that keep each blade at proper pitch.
Simply attaching big wings to a heli won't let it fly this way for long, but special craft that take off as helicopters and fly like planes can and have been built.

Not to mention yaw control which becomes interesting. Not sure how much ironic answers are appreciated here, generally, but I like it...

But if the rotor wings could be designed in such a way that their length can be adjusted&reduced for the vertical flight phase, then increased back again for landing, then the drag problem could be overcome.  Further if the aircraft legs could transform into 2 horizontal stabilizers for horizontal flight 90 degrees of the 2 that will act as vertical stabilizer equipped with a rudder.  The fourth leg at the bottom could  extentend into the one at the top to extend it into a tail.  It could be designed with 3 legs that are 90 degrees to each other, one with the ability to extend into a rudder

It would be far simpler just to add wings and tilt only the rotor 90 degrees, and not the entire fuselage. Drag would be much lower, and the occupants wouldn't have to endure hanging down in their seats.
You could call it... a tilt-rotor.

You could.  But in the advent of tiltrotor aircraft why would you want to?
What you propose is grossly inefficient, awkward for the occupants and counterproductive in terms of drag.

What about now?

Again, the idea has been considered but there are simpler alternatives such as a tilt rotor design.  Other types like X-Wing concept aircraft have been investigated as well, but they have their own drawbacks that make the design prohibitive, too.

This picture of the BV-37 came up that somewhat proves my concept, but the wing moves forward in flight and the helicopter leans onto the wing some. I wanted to know if the wings span was long enough if the rotor blades could serve as a propeller and the wing provide lift.


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What rotors would be best for atmospheric reentry?
What rotor blade set up would be the best for entering Earth atmosphere or to shed speed before landing in place of a parachute?
I understand that the longer blades are more efficient, but are not as strong.
The link below explains how this might be used.
https://space.stackexchange.com/questions/35699/could-a-rotary-rocket-charge-a-super-capacitor-while-descending-on-mars's

https://commons.erau.edu/cgi/viewcontent.cgi?article=1283&context=discovery-day
Related: Would an aircraft with contra-rotating propellers longer than the plane's wingspan be able to fly?

At hypersonic speeds, I doubt the blades would survive the pressures and shear loads of reentry, much less the heat.  I don’t think this idea would work at all.

According to the article from which this graphic comes, the rocket will launch to 2,7000m. It's not clear whether that means 2,700m or 27,000m, but either way it's in atmosphere, so no re-entry. SpaceX currently land their boosters using a reserve of rocket fuel. It's difficult to see how the weight and complexity of rotor hub, rotors and two parachutes would improve on that.

You couldn't do it to control reentry, but you certainly could do it for a vertical landing system that might be a bit less technically demanding than a rocket that can descend under control with its engine running as SpaceX does.
You are limited by the mach# of the tips of the rotor, which have to be subsonic while spinning.  You would want a rotor that can fold alongside the rocket and deploy and accelerate to operating RPM once it's approaching the surface, and reduce the descent rate to an acceptable level where the craft can land with its gear absorbing the landing shock in a zero forward airspeed autorotation.
This is not hard to do with a well designed gear and there are gyrocopters with gear that is designed to handle zero-forward speed autorotation landings (basically, they can cope with high descent rates by using long-stroke shock absorbers).
The rotor would simply be the simplest, lightest, cheapest system that you can come up with, that the landing gear can handle.  If you could do it with two blades you'd go with that, but maybe packaging would require more blades, and you'd have to find some sweet spot with the best overall landing performance between the rotor and landing gear, and your physical packaging limitations.


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What is the best way to navigate a storm?
Let me say I had no choice other than fly through a storm I'm on a one way mission to airdrop a vital cure to an isolated area. I can't turn around, land or go around, I have to go through. Going around it means I run out of fuel and they all die.
How does a pilot navigate a thunderstorm? What altitude has the least lightning, wind and hail?

It's hard to think of a realistic case where flying through a thunderstorm is your only option.

@GdD your on a mission and you got to drop them bombs on time.

Um, no @Muze, you'd fly around it. Flying through a thunderstorm would be a great way to have your plane damaged enough you can't complete your mission.

@GdD, your plane is overloaded and you really want to go further than the destination you had filed in your flight plan just to satisfy the mandatory fuel margin. (see AF447 on May 31, 2009)

And that worked out great @bogl!

@GdD Not at all. The point is that there are more pilots dumb enough to try it than you would like to believe. Some of them hold an ATPL.

"Going around it means I run out of fuel and they all die."  Fly into it and you may die also, never to help another group in the future. How is that any better?

You are flying a research plane and collect data about it ;)

See this video and countless others re hazards of flying in thunderstorms- -https://www.youtube.com/watch?v=83uvKWJS2os

If you're flying through a thunderstorm, lightning is the least of your concerns. Airplanes are struck by lightning all the time, and while it may mess with your electronics (navigation, radio, etc.), it very rarely causes any more damage than that. But to answer the second part of your question, there's no one place in a thunderstorm that has any more or less chance of lightning than any other.
The big issue with flying through a thunderstorm is with the wind. There's a lot of vertical air movement in thunderstorms, and the wind shear can easily cause structural damage to your aircraft if you're not very careful (and sometimes the wind can damage or destroy aircraft even if you are careful. And that's leaving out other dangers, such as hail. Thunderstorms are not good places to fly).
The accepted method for flying in a thunderstorm is to fly straight through (as turning increases the load factor, making a stall more likely) at less than your maneuvering speed (to minimize the chance of structural damage to your wings), accepting any changes of altitude (again, to minimize stress on your wings).

Hail damage is also a big concern, if you survive the severe updrafts and downdrafts.  Give them a wide berth.

Read the book "Fate is the Hunter" by Ernie Gann.  One of the great aviation books.  Gann was flying DC-2s and 3s with, IIRC, TWA, in the mid 30s.  The latest nav technology available was the ADF and the AN range.  Weather radar?  What as that?  Cells in the way?  They would just blunder through storms, hanging on for dear life, using towels to try to limit the water leaking through the DC-3's windshield and windows.

Good point, @CrossRoads, I'll add something about hail to my answer.

"there's no one place in a thunderstorm that has any more or less chance of lightning than any other."  That seems highly unlikely.  https://www.weather.gov/media/pah/WeatherEducation/lightningsafety.pdf If "intra-cloud" is more common than "cloud to ground" I think it would be reasonable to say there is less change of lightning if you fly near the ground.

Boeing has a handy graph:  https://www.boeing.com/commercial/aeromagazine/articles/2012_q4/4/  Presumably there are fewer lightning strikes above 15,000 feet in part because it is easier to avoid storms during the cruise phase.

You DO have other choices than to fly through a thunderstorm, or trying to land in one.
It has been mentioned, and I will repeat again, thunderstorms present great danger with wind
shear, hail, lightning, and very strong vertical winds.  They are to be avoided.
The greatest danger is their unpredictability.  In a matter of 1/2 hour they can blossom into the severe category, which will be dangerous to any type of aircraft.
Also, be aware that radar information may not be real time, and in unstable air an apparent opening may close rapidly.
Regarding the scenario presented by the OP,  these options can be considered:
Fly over the top of it.  Severe thunderstorms can reach over 50,000 feet. Difficult.
Land, tie your plane down, and wait for it to pass.  A thought, especially if there is a line of storms approaching.  A few hours delay may save the air crew's lives.
Fly around it.  This is the preferred option.  Carry extra reserve fuel and be aware of the weather.
Important to realize thunderstorms are creatures of vertical airflow.  Hail, turbulence, lightning, heavy rain can occur in any part of the storm.  Because of the vertical nature of the airflow, strong downdrafts (microbursts) can crash a low flying plane, and turbulence can severely stress an airframe.  Softball size hail and tornadoes are a testament to the forces involved here.
However, not all thunderstorms are super cell monsters like the one pictured.  If the plane is built to handle it, with proper information and approval, it may be a "GO".  But not without risk, which would also include icing.

I agree wholeheartedly, but this doesn't answer the question @RobertDiGiovanni, it's really a comment.

Safely flying around a super cell as pictured would involve a radius of around 40 miles.  The detour is there for roughly 120 miles.  A good rescue plan would not include this scenario (don't forget how hard the Air Force works on logistics (air to air refueling)).  Smaller storms containing lightning are commonly flown through, NOT THAT ONE.  A rescue pilot would be on the radio asking for options (and using all available resources to complete the task).  Getting boxed into a situation where you must gamble, and have no options, is poor.

I think it is a good answer and adds to the body.


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What are the limits of ground effect over water on an airliner plane at cruising speeds?
What height over a fixed plain (such as water) can a plane begin to take advantage of the ground effect or when air is compressed between the wing and ground near cruizing speed? Could an airliner type plane fly above the water for an added lift for range in an emergency where a plane may lack the power to climb?

Does this answer your question? How high does the ground effect last?

Do you mean "fixed plain" or "fixed plane"? Either way, what do you actually mean by that? In English, a "plain" is typically a wide expanse of flat land, generally characterized by grasses and low shrub vegetation. It's not clear to me what that would have to do with "above the water".

There's an entire type of vehicle called a "ground-effect-vehicle" that takes advantage of this effect. Yes, an airliner can do it, so can a small plane. Many early Atlantic crossings used ground-effect to increase range.

@Peter Duniho, the "such as water" modifier means he was talking about a fixed "plane", and not necessarily grasslands.  It was a typo, not difficult to discern meaning from the context.  (because in English, "plain" can also mean "simple or ordinary"...)

@PeterDuniho it does not.

As noted in the answer linked in comments, the practical limit for ground effect is about half the wingspan over a relatively flat, impermeable surface (you'll get better effect from smooth water than from heavy swell, for instance).  Flying in ground effect for either economy (reducing power needed to fly) or in an emergency (engine out prevented climbing, but allowed ground effect flight for considerable range) has been done many times, and there have been vehicles designed to fly almost entirely in ground effect (ekranoplan was the Russian name for these).

Ground effect only comes into play within about 1 wingspan’s height above a surface and it is largely unnoticeable until approx 1/4 to 1/10 of a wingspan above the surface.
While it is true that ground effect greatly increases the lift to drag ratio of the wings, it does not reduce parasite drag nor does it account for the fact that it is more efficient to fly at higher altitudes where the atmosphere is much thinner for greater speeds per unit of fuel consumed.  Flying low to the ground is dangerous as there is less margin for error if something goes wrong and would limits operations to day VFR conditions.
Transport airplanes have been test flown and certain flight profiles have already been determined to offer the greatest range.  These occur at altitude and on reduced power settings.
Heavy sea transports which to use wing in ground effect for lift  have been proposed in the past, but been found to be in efficient compared to existing jetliner designs. WIG craft have been used for short transport missions like island hopping, where that kind of a craft would be faster than a ship, and more efficient than a light airplane.

What height over a fixed plain can a plane begin to take advantage of the ground effect or when air is compressed between the wing and ground?

It varies, but, generally speaking, ground effect usually becomes noticeable somewhere around half the plane's wingspan above the ground. So, for a plane with a 50-foot wingspan, ground effect would only be viable below 25 feet.

Could an airliner type plane fly above the water for an added lift for range in an emergency?

Theoretically, yes, but it's unlikely to be very helpful.

Flying that low is dangerous in and of itself. Even out over the water, there are boats, buoys, rogue waves, breaching whales, etc., etc., etc. Not to mention, you have to be very careful when you turn not to dip your wing in the water, and even a slight downdraft will kill you before you have a chance to react.
Fly high enough, and the reduced air density (and subsequent reduced drag) can increase your range more than what ground effect would give you.
Ground effect is highest over a smooth surface, so if there are waves, you won't get as much drag reduction as you might be expecting.

That all said, it is possible, and has been done several times.


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At what altitude does a wing become useless?
At what speed or altitude does an aircraft become more efficient as a rocket then a plane using thrust vectoring instead of the wings to steer? At what altitude did the flight control surfaces of the Space Shuttle become useful again or X-37B is still useful?

Source: http://www.xairforces.net/newsd.asp?newsid=817&newst=7#.XCbNY7hhrwc
Related: https://space.stackexchange.com/questions/33152/could-rockets-launched-from-the-ground-use-wings-in-the-stages
Is GOCE a satellite or aircraft?

Maybe wings never mattered...
Altitude:
The answer depends a bit on the design of the craft in terms of control surfaces/wings. Atmospheric reentry begins at the Karman line at an altitude of 100 km (62.14 mi / ~ 54 nautical mi) (327,360 feet) above the surface. Which is generally when air resistance starts to matter but that does not mean control surfaces will do anything at precisely that altitude. In terms of practical vehicles that have been flow:
The space shuttle starts maneuvering with its aero surfaces ~30 miles up

At an altitude of approximately 30 miles [158,400 feet/50km], the orbiter makes a series
  of maneuvers and S-turns to slow its speed. At 9.5 miles in altitude
  and at a speed of Mach 1, the orbiter can be steered using its rudder.
  The on-board computers fly the orbiter until it goes subsonic (slower
  than the speed of sound: Mach 1). This happens about 4 minutes before
  landing.

Neil Armstrong famously flew the X-15 high enough to need the reaction control rockets to keep from skimming off the atmosphere, he was up around 207,500 feet.
So somewhere up around ~175,000 feet, practical control surfaces start to work. Its possible aircraft could be built to get above this region but they would need the ability to either fly up there or survive the decent which is a whole different can of worms.
Speed:
Any speed so long as your thrust to weight ration is greater than 1. All rockets start from a speed relative to ground of zero. Rockets simply have engines capable of providing 100% of the thrust needed to move (generally directed in the up direction). Planes have enough thrust to overcome drag then use the wings to help them with the lift (up direction) portion of the problem. There are aircraft that have a thrust to weight ratio greater than 1 and are capable of "flying" in a way without wings.

Re flying with a thrust-to-weight ratio greater than one-- with wings-- some of the content under the heading "Powered climb at 45-degree climb angle at 8 different ratios of Lift to Drag:" in this answer may be of some interest -- https://aviation.stackexchange.com/questions/40921/does-lift-equal-weight-in-a-climb/56476#56476

"up in the ~175,000 feet practical control surfaces start to work" start?

Speed or altitude is relative. To me the answer to this question is "At whatever speed or altitude the flight control surfaces cease to be effective due to insufficient relative wind or atmosphere and only thrust from the propulsion system can have an effect on the craft."

Right, for example the wing isn't doing much when a rc model ducted-fan jet hovers, even if only a few feet off the ground. So it's really not even always a matter of insufficient atmosphere.


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What is the largest and heaviest payload ever flown?
Not counting the plane what is the largest or heaviest payload ever flown?

Related

[Scaled Composites Stratolaunch can lift 250 t and it is a flying aircraft. It, however, has not been tested to this capacity yet.

Source: Popular Mechanics
The Antonov-225 currently holds a number of records...

Heaviest maximum take-off weight at 710 tons (1.4 million pounds)
Total air lifted payload of 558,590 pounds
Single item payload of 418,830 pounds
Longest wingspan of 290 feet for any aircraft in service (Stratolaunch and H-4 Hercules had larger wingspans, but neither are currently in service)

The A380 by comparison has a maximum take-off weight of around 1.2 million pounds (600 tons) and is 260 feet wide.
For helicopters, the heaviest payload holder currently goes to the Mil Mi-26

Source: Wikimedia
It has the world record for a payload lifted to 2000 meters (about 6500 feet) of 125,153 pounds (about 62 tons).

As for the "largest" I'm not sure you can separate that. The Antonov 225 holds the largest single item payload referenced above, so that is held by weight. If you are asking about physical size, that could mean a number of things. Longest, widest, tallest, etc.
The Airbus Beluga held the world record for the most "voluminous" cargo ever lifted when it transported a chemical tank for a marine vessel.

You’re missing the space shuttle carrier. Probably more voluminous than the beluga’s cargo

@Antzi It may be close, by volume the shuttle may be a little bigger, but it is hard to calculate due to its irregular shape. It could also be argued that the Antonov 225 also holds that record, but you'd have to compare the volume of the STS to the Buran. The STS is heavier, but the two shuttles are very similar in size.

https://en.m.wikipedia.org/wiki/Stratolaunch_Systems has a longer wingspan

@Muze True, it was only recently that it flew, and it may never fly again since the company is now defunct and looking for buyers for its assets. The Stratolaunch was a very specialized vehicle and will probably be scrapped for parts. The H-4 Hercules also had a wider wingspan, but never really "flew".

@RonBeyer thanks. https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Giant_planes_comparison.svg/260px-Giant_planes_comparison.svg.png

Yes, you can separate largest from heaviest. A tank (armored vehicle) is heavy but small; a rocket fuel tank is large but light.


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Could this helicopter's body be a flying wing?
This question: Could a helicopter fly like a plane this way? asks if a helicopter can turn sideways using additional wings to tip forward to allow the rotor blades to act as a propeller. The tail-setter and other types of planes came up as answers. Similar to the tile Setter their first function of this aircraft would it be to hover then to fly as a plane.
Would this model helicopter with a body shaped of a flying wing be able to fly at a 90 degree angle and at a 90 degree twist transferring lift from the rotor blades to the body?
I have sketched out a picture below on how on might and apologize for imperfections. The body could be longer to create more wing span.

The pilot would need a special seat or controls to adjust for tilt but they would enter it like a normal helicopter. Could this model fly, keep maneuverability and feel of a helicopter foremost?
The pilot would only need to dive Rotor head first to allow the body to serve as a wing.

Have you seen the Boeing V-22 Osprey?

Can you include the annotation for the numbers in the images? It is not really clear what the various lines are supposed to represent.

@JanHudec number of wings

You talk of a model, which is by definition unmanned, and of a pilot. Which are you really asking about?

No, you need sufficient wing span to produce lift without excessive induced drag.
Lifting bodies are only a thing of hypersonic flight where the induced (component of wave) drag is tiny compared to the form (component of wave) drag.
Induced drag is an unavoidable side-effect of producing lift over a finite span. It decreases with wing span, and with dynamic pressure (which in turn grows with density and square of speed; indicated speed is a square root of dynamic pressure). So at slow speed, you need considerable wing span to generate lift efficiently. This applies also to helicopter rotors, which is why they have such large diameters.
Since the other component of drag, form drag, increases with dynamic pressure and cross-section, the optimal wing span decreases with speed. But even jets still have quite high aspect ratio (square of wing span to wing area). Flying bodies without much any wings only become efficient well into supersonic area—and keep in mind that anything resembling a propeller (including tiltrotor) is limited to subsonic.
(At supersonic speeds, the mechanism of drag creation is different, called wave drag, but it still has a form component and induced component and the induced component still depends on wing span and dynamic pressure in the same way.)

What would you do to improve on this design?

@Muze, it's not possible to work with a design without specified goal—and most mission profiles have pretty good designs covering them already.

The goal is to fly like a wing at higher speeds that far exceed any helicopter today.

@Muze, I could imagine something like an airplane with a foldable rotor. Basically similar principle as F-35, but a foldable rotor instead of the fan to reduce the induced power in hover and therefore increase payload fraction. The reason for folding the rotor is that you can't use it for propulsion at higher speeds, because the tips would go supersonic, and without being stiffened by the centrifugal force the blades are quite fragile, so you'd have to at least align them with the flow to reduce the aerodynamic forces on them. Still subsonic; above the usual M0.85 nothing is efficient.


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Is it better to have the engines center or apart on a plane?
Are there any advantages or disadvantages to having the engines closer or farther to the center of the plane?
How is the ideal engine spacing determined length or span wise for performance?
Related: How is the center of mass controlled in planes with multiple tail engines?

The main reason they are usually kept close to the fuselage is probably their weight that the wings have to carry.

Hmmm. Weight? As far as I learned here, placing engines on the wing reduces the load on the wing, which is created through the lift. Even on two engine planes the engines are separated from the fuselage by a quite large distance.

Further out -> reduces wing bending in flight, but increases wing bending and also obstacle collision risk (signage etc.) on ground. Also makes asymmetric thrust harder to control. May reduce fuselage influence on engine intake flow.

For military aircraft like the A-10, having the engines separated from the body helps in situations where the engines “Go explodey on you”, in the words of an A-10 pilot.

Lengthwise or span wise?

@CptReynolds could you put that in an answer?

@Koyovis both.....

@Muze I didn’t put as answer because it doesn’t really answer the question as asked, it’s just a list of pros and cons, and not a comprehensive one either.

What are your criteria for "better"?  E.g. putting engines on the centerline eliminates problems of asymmetric thrust, but means you have to mount them where they're more difficult to access for maintenance...

the further apart the engines are, the worse becomes the yaw in an engine-out situation. but spacing them (and their fuel tanks) further apart along the span of a wing allows more even distribution of load stresses.

edited just now.

Ideally you would like the engines as close to the aircraft centerline as possible to eliminate asymmetrical thrust loading caused by failure of an engine.  In multi-engine aircraft, this is not always possible to do due to fuselage configuration, empennage, fuel tanks, wheel wells, etc in the wings.  When multiple engines are all turning and burning, there is little difference between a centerline thrust twin and a conventional twin.  But the difference becomes problematic in the event of an engine failure.

One of the mysteries in general aviation is that twin engine airplanes have more accidents than singles.  One of the factors is asymmetrical thrust after an engine failure.  As such, centerline thrust is considered to be a virtue because pilots have an easier time with it.  It also allows all the engine's power to be used for thrust, instead of wasting some on rudder.
So the closer to centerline the engine is, the easier it is on the pilot.
However, the nature of propellers makes it difficult to put two propellers on centerline.  You either need a "pod" fuselage with a twin tail, like a Skymaster... or stacked contra-rotating props with one engine per prop. Of course, the trend toward electric props is going to improve that, allowing prop placement now not possible.
Of course in complex aircraft and jetliners, the autopilots relieve much of the workload and opportunity for error.  Those will usually be there to assist, because there's little chance of those automated systems also failing at the same time.  That reduces the safety impact of widely separated engines.

in case of an engine-out in a light twin, you can always rely on the second engine to deliver you precisely to the crash site!


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Would a long pole help stablize a jet pack?
Jet packs are classically worn like a book-bag on the back. Would it be more stable if the exhaust ports were separated? Would the jet pack be easier to control if the turbines were at the ends of a long hollow pole that would also hold the fuel? Would a pole help stabilize hovering like it does a tight rope walker?

What has the image got to do with the question??

@Jamiec is this better?

Read about first Goddard experiments with rockets, he quickly discovered putting the exhaust above the structure don't improve stability

No. it doesn't. You've been warned before about irrelevant large images in your questions.

Not really.
First the reason why it could be absolutely terrible: a long pole is flexible, and you could run into awful oscillations. A long pole with two heavy masses at the end will have a very low resonance frequency, and probably also some interesting torsion effects.
The reason why you would want a separation is simple high school physics. You control your attitude with torque. A longer arm would mean a larger torque for the same thrust angle deflection. It would be a good solution if the thrust vectoring angles would be excessive otherwise.
From what I gather, the opposite is true. Minute adjustments cause a big change in attitude, and the system is already quite skitterish. The only use case for your system may be avoiding burnt legs.
Your idea of a hollow pole with fuel in it wouldn't be half bad though - it might help your jet pack operator balance, just like a rope walker. Just don't put jets on the end of the pole.

Updated the question.

I don't think that much separation would help with control. See this article on actual "Iron Man" flying suit.
https://www.cbsnews.com/news/real-life-iron-man-suit-now-available-for-sale/


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Could a BV-347 be upgraded by extending the wings?
The CH-47 Model 347, also referenced as BV-347, was a helicopter that utilized a wing for forward flight and to carry external payloads. Could the wings be extended and engines on the rear rotor modified to also provide enough trust to provide enough lift from the wings alone?
The wing would be up during takeoff and landing. Extending them with modern material should not be too hard, but modifying the turbine for thrust vectoring may be costly.
related: Rotor head plopper upgrade

If you add more weight, at some point you‘re going to lose hovering and vertical takeoff/landing capability, at which point it becomes an aircraft - and not a particularly optimised one either! I don’t see why it couldn’t be done in principle and if willing to (engineering-wise and money-wise), but why should you?

I realize the proposed concept is nuts, but I don't quite get the downvotes.

The meaning of the phrase "engines on the rear rotor" is unclear and seems to suggest a misconception.   In reality, both engines drive both rotors.

@quietflyer rivised thanks.

In an effort to find the most realistic solution to an odd proposal:
The wing:
It does seem possible with current materials to extend the wing and keep it structurally sound, although the design would likely have to be altered a bit, probably with an external strut in order to have a wing root thin enough to keep the underslung weapons. Additionally, a much longer span would inevitably increase your root chord if you want to keep sufficient wing area, and I have a feeling that would start to block a lot of the area under your rotors, decreasing your vertical thrust and your payload capacity.
Without additional control surfaces (probably on the tail), the wing would also create large pitch instabilities, requiring the rotors to spin at all times to keep positive directional control. They could probably be spun at low power, but this would still limit your forward thrust component since some power would still have to go to the rotors.
The engines:
While AEhere mentioned that the turboshaft engines aren't designed to provide jet thrust, but rather spin the turbines and thus the shaft, a dual-mode design could be feasible that would direct high-energy air proportional to the needs of each propulsion system: to the shaft turbines for vertical flight, and to jet nozzles for forward flight, including potentially into another set of turbines powering a turbofan for better efficiency. This seems to me like the most efficient way to keep the vertical lift system powered at some stages of flight while also providing sufficient forward thrust in others, as well as allowing mixing for partial thrust of each in between stages.
More ideas to come?

In principle, if you have money, time and nothing better to do, yes, but it would be a Frankenstein's aircraft that would be a terrible  plane and helicopter at the same time.
Wings on helicopters have been used on a number of models, most notably the Mi-24 family, to provide cruise lift. They work as expected for short low aspect ratio wings and serve a dual purpose to house the weapon hardpoints. The large wings you draw would require massive reinforcement of the roots and lead to stability complications, see below.
The engines would be a different story. The turboshafts on the Chinook are optimized to not do what regular jet engines do: provide thrust. Their exhaust has little momentum because on a regular helicopter engine that serves no purpose, the energy needs to go into shaft power to drive the main gearbox. On a turbojet engine, the energy is used to accelerate a mass of air out of the nozzle.
From a stability point of view it would also be terrible, as the wing by itself would be unstable in most weight and balance conditions and require the rotors to keep providing torque in flight to keep the airframe pitch-stable. And if you need to keep the rotors spinning throughout the flight anyway, then why add a wing large enough to lift the entire thing?


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Would a Chinook helicopter fly better with contra-rotating along with counter-rotating propellers?
Would a Chinook perform better or have any advantage with dual contra-rotating propellers over a Chinook with just normal counter-rotating propellers?

There seems to be a lot of confusion around this question. First, it's not clear whether you mean contra-rotating (two rotors turning in opposite directions on the same axis) or counter-rotating (opposite directions on different axes). The Chinook does have counter-rotating rotors, but not contra-. It's not clear what your image shows or where you got it from. It appears to be some sort of model: does it have contra-rotating blades?

@DavidRicherby yes contra and it does have contra and not counter.

@Muze I thought the question was clear and have edited my answer to try to avoid suggesting otherwise.

The Chinook DOES have contra-rotating rotors, in a sense.   The front and back rotors turn in opposite directions.  (Actually, the correct term for this is "counter-rotating".)  Read more on this elementary physics page about torques and angular momentum-- https://learn.parallax.com/tutorials/robot/elev-8/understanding-physics-multirotor-flight/rotation-torque-and-angular-momentum
Of course the question was clearly asking about true contra-rotating rotors-- two rotor disks rotating in opposite directions on each rotor mast.  This might have a few advantages-- like allowing the Chinook to tolerate an even greater fore-and-aft variation in CG-- but would obviously introduce a great deal more complexity.  Whether it would fly "better" is hard to say-- the energy lost through the additional gearing, as well as the energy expended to lift the extra weight, might not offset any theoretical aerodynamic advantages.
Here are some interesting additional tidbits of information about the Chinook--

There is no fore and aft (cyclic) movement in either disk. (There can't
  be, they'd strike each other.) Aircraft pitch is controlled by
  differential collective between the disks.
Roll is produced by lateral (cyclic) in both disks simultaneously, and
  yaw by differential lateral cyclic, if you follow my terminology.
Actually there is a slight fore and aft input by a system called LCT
  (Longitudinal cyclic trim) that tilts the gearboxes fore and aft to
  reduce excessive deck angle at speed, but it is not part of the
  primary flight controls. Conventional flight controls produce these
  rather unusual inputs via a very complex mechanical mixing unit in the
  broom cupboard behind the P1.
Due to the dynamics the Chinook develops maximum translational lift in
  sideways flight to the left, hence the antics of the logging machines
  plunging sideways on the end of their long cables - gives them greatly
  increased lift capacity.

Source of quoted material above-- this post (#2) in another forum-- https://www.pprune.org/rotorheads/163538-chinook-other-tandem-rotors-discussions.html#post206883
Note that yaw control is NOT dependent upon the difference in the torque applied to the front and back rotors.  Therefore it's clear why this unique and interesting design could tolerate a large variation in fore-and-aft CG, even without the added complexity of a pair of contra-rotating rotors at each rotor mast.  In fact, since the rotors are mechanically locked together (see below), it must often be the case that one rotor is absorbing more torque from the transmission system than the other rotor.
A few more bits of interesting information about the Chinook--
The rotor blade systems, though mounted on masts of different height, apparently sometimes do actually intermesh in flight.  However they are mechanically locked together by the transmission gearing so that they are forced to turn at the same rpm so the individual blades cannot strike each other.
(This may call into question the statement above that it that it would not have been possible to design the Chinook control system to include collective pitch inputs to either rotor because the blades would strike.)

The difference in height of the rotor mast heads is indeed quite
  noticeable when you take the fuselage top as a reference. However,
  in-flight images show that the fuselage itself is tilted up in slow
  flight (as well as on the ground), so a part of the difference is
  already gone then. Most of what remains is further eaten away by the
  difference in rotor mast attitude: the shorter mast of the apparently
  ‘lower’ front rotor is tilted forward, away from the rear rotor mast.
  The front rotor mast is tilted towards the nose, and the rear rotor
  mast is also tilted slightly forward, but less so than the front one.
Now add the real life variations of the actual blade height when
  passing over the fuselage top due to cyclic pitch and ‘rudder pedal’
  inputs (blade flapping) and turbulence (blade flapping and flexing).
  And factor in the considerable blade droop (downward bending) when
  parked and on initially setting the rotors in motion.
I hope my contribution counteracts the inevitable legend of the often
  mentioned apparent difference in height of the two rotor discs: in
  real life the two rotor discs mesh more or less all of the time. The
  mechanical synchronization is indeed the crucial element, just like in
  the egg beater. The mast height difference and the related fuselage
  attitude are there for other reasons.

Source of quoted material above-- second answer on this thread-- https://www.quora.com/Why-dont-the-Chinook-helicopter%E2%80%99s-rotors-collide-with-each-other -- Answered May 3, 2018 by Frank Sturm, former Flight Ops. Engineer at Martinair Holland (1979-2003)

Just for added interest here a link to a maintenance/ troubleshooting manual for the Chinook-- http://www.chinook-helicopter.com/Publications/AMTFH/CH-47_MTF_1_October_1968.pdf

I think the question is asking about actual contra-rotation, not using the wrong term for counter-rotation.

Agreed, was kind of making a joke or being a smarta**, have edited to not suggest question was using wrong terms.

No.  The RC model there is using counter-rotating rotors because the upper rotor with the stabilizer bar has a specific function that makes the model controllable in the first place.
The Chinook already benefits from built-in anti-torque control so that would be a waste of time and oodles of money on the real one.
Fun fact about the Chinook: it can climb better, and fly on less power overall, going sideways. Both rotors are able to bite clean air, achieving translational lift, when going sideways.  When going forward, the rear rotor has to cope with the wake of the front one.

Do you mean counter-rotating or contra-rotating? The real Chinook does have counter-rotating rotors (one turns clockwise, the other anticlockwise).

First, a little nomenclature: Contra rotation is opposite rotation on the same axis, as seen on the Tu-95 bomber.  This increases the efficiency of converting horsepower to thrust in a prop design.  Counter rotation is two props spinning on 2 DIFFERENT axes, whose torque effects cancel, as seen on the P-38 Lightning.
The Chinook benefits from counter rotating props (on 2 different axes) as a cargo carrier because it can adjust its center of lift to match the variations in CG of different cargo loads.  Moving one prop under the other would lessen this advantage, but mounting 2 sets of contra rotating props would be possible.
May be a possible, but not needed change.
As far as flying better sideways (going Osprey?), a great point by John K.

Interesting tidbits about the Chinook-- see added quotes in my answer--

This post can be summarized in just one word, that is "Maybe".

"The Chinook [...] can adjust its center of lift to match the variations in CG of different cargo loads" Can it? Aren't the rotors mechanically linked so they keep the same rpm? Setting different collective pitch values on them will yaw the airframe, so any "lift adjustment" would be via cyclic trim, like any other helicopter.

Yes, they can!  See tandem rotors on Wiki.  Imagine the tail rotor migrating to the top like the eye of a flounder as it grows.  The tandem works more like a wheel barrow than a unicycle for varying CG loads.  So you increase collective on the heavy side and use cyclic to control the resulting yaw.

I honestly don't get your similes most of the time and this is no exception, but FM 3-04.203 explains it nicely and you are right about yaw control in tandems, at least American-made ones. I was thinking about coaxials since I am more familiar with them.

@AEhere the issue for the full scale seems the be the intersecting prop arcs.  I would imagine another interesting question might be "would a single co-ax lift more/be more efficient" as the blades may be longer.  I'll work on the similes.


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Could a rotor head wing be plausible?
Could a helicopter support a detachable wing with no controls on top of the rotor head in place of a radar dome? Could the rotor head be made to support more weight? Would it be more fuel efficient at higher speeds, add a level of redundancy, or increase speed?
I did consider clearance and the drawn pictures is not to scale. The wing may need to be drawn longer, further back or placed higher. This would be a payload in itself for extending the range of the helicopter under lighter loads.
A second feature is that the wing could be made to catch the vortex left by the rotor blades in flight increasing air pressure under both the rotor blades and rotor wing.
Similar to the way ducks fly in a v formation the wings would also take this advantage from other helicopters with this extra wing for extra lift.
Could extra lift be created from the rotor wing, the vortex ring redirection and /or flight in a V formation?

Why though? This looks like a solution in search of a problem.

It's difficult to see how you'd maintain sufficient rigidity in this wing arrangement to keep it clear of the rotor disk. By the time you've added sufficient structure to do that you've added so much weight that the helicopter's payload is reduced to uneconomic levels. Since the principal reason for having a helicopter is its VTOL capability, and your wing only impedes that, why would anyone bother?

@AEhere why is included now thanks. VTOL would not be affected.

Interesting idea! I'd like to see how the wings would perform with the increased pressure from the vortices. Maybe it would be comparable to a normal wing with ground effect?

@Pheric thanks my mind says yes, but I can't prove or disprove it on paper. Hopefully some one can.

It seems like you're trying to get a free lunch by extracting work from the vortices. It feels like blocking them should increase the pressure below the rotor disk, making it harder for the rotors to generate lift by forcing air downwards.

@DavidRicherby not a free lunch, just recovering energy lost from the vortex ring, but that would be a bonus. Primary lift would be from forward travel..

@Muze Right, so given that lunch isn't free, the question becomes how much lunch costs and whether you'd be better spending your lunch money on something else.

There is no need to attach the wing to the rotor head - simply attach it to the top of the fuselage. You could run the rotor through a hollow tube and attach the wings to that - the inner uncovered section of the wing is an interesting idea.
If you are looking for VTOL, a tilt rotor with two electric motors driven by the gas turbine may be a better option.

I thought this as well but as the bottom picture show if it was attached lower the wing would not take advantage of the vortex redirection.

Surely a generator, two electric motors and wiring would be heavier than gearboxes and shafts.

Maybe not so heavy: as this prototype shows: http://evtol.news/2019/02/25/electric-vtol-for-organs-on-demand/

@stackex555 the prototype electric R44 referenced in your link traveled 1/10th of the typical range for the type and set a record doing it.


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Why is this placard on a Boeing 737 NG?
I was working on the cockpit of a Boeing 737 and was intrigued when I saw this placard. From what I know this has to do with Citibank being a lender or something along those lines.
Why would this be placed on the plane? Is there some kind of regulation that covers this kind of topic?

Was this on a working plane, or a display plane?

@DJClayworth This was on a working plane.

It's more likely that the lending bank requires a placard be "durably affixed" to a secured asset in their loan agreement, than any aviation regulation.  In the US, a court or creditor trying to repo a plane would have access to information about the aircraft's actual ownership by other sources.  But perhaps the idea is also that a commercial bank in, or the government of, Crobania, taking a collection action against the airline, would see the placard during a repo inspection and say "crap, the airline doesn't own this airplane, so seizing it is a losing proposition and a waste of our time."

See this definition.

Was it a cargo aircraft? And if so it may be a part of a fleet owned by another company who offered the aircraft(s) as collateral to CITIBANK.


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UH1 fin under chin bubble
What is the purpose of the fin under the nose. It’s close to the co pilot side and starts at the bottom of the battery compartment and ends Near the bottom of the chin bubble.
Wikipedia Photo

@757toga, I don't think the ring is actually part of the fin being asked about though, because you can see the same paired ring on the near side.  I think it's just the perspective that makes it look like part of the fin assembly.

It’s a wire strike protection system.
https://en.m.wikipedia.org/wiki/Wire_strike_protection_system

The question is about the “fin” that is flush with the bottom, not the wire cutter sticking out an angle.

I added another picture trying to remove the wire strike system so it’s not as confusing

Then I don’t think “fin” is a good choice of words, because I think shark fin, and also went right to the forward angled protuberance. The scribbles don’t help, neither does the same view highlighting the forward angled bit.  If what you are looking for is what is in the third photo, I recommend simply deleting the others.  They do nothing but add confusion.

Ok the other two pictures are gone. Mr hall the picture that is still there, do you know what that is for?

Anthony, unfortunately I do not, otherwise I'd answer.  But... when I first read it I was about to post this same answer because I too looked at the "wrong" part in the first photo.  Thanks for clearing it up, hopefully somebody will know.

Ok thanks for the feed back, if you need up finding it out please let me know. I will try to get a better picture Tuesday and put it on here.


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Vertically moving wings
Would there be a benefit to vertically adjustable main wings?
I recently watched this anime series and got curious if the wing moving mechanism could have any plus points, or if it was likely just purely for the rule of cool. Being able to be stored in smaller spaces does not count.
The aircraft is yukikaze from Sentō Yōsei Yukikaze.

edit: a second image where the wings are down to show the difference

Do you have a drawing that depicts the moving wings? I can't recognise them in the third-angle projection you posted.

@Bergi if you compare the top right depiction of the aircraft on the top image and the aircraft on the bottom image you will see that the wing can bend up and down

If you ever wonder what different types of wing motions are good for, look at birds. They use all of them.
In this case it reduces lift while keeping drag the same. Birds use this when landing. It also adds dihedral.
For our much larger aircraft, we use spoilers due to scaling issues.
Many, if not most or all, motions involving the entire wing can be used for control in place of things like elevators, ailerons, flaps, spoilers, and rudders. But they have more value at smaller scales where structural scaling causes strength of materials to no longer be an issue and where aerodynamic scaling causes the conventional control surfaces to be too small.

The XB-70 had wingtips that folded down in flight so it could use the shock wave to create additional lift.  However this particular approach (compression lift) only applies to supersonic airplanes.

(picture from Wikipedia)
I can't think of any other planes that allowed any part of the wing to move vertically for aerodynamic reasons.

the weird thing is that the wings get angled upwards, and seem to stay in that position at all times in the air.

@NajLitarvan Well, it is fiction after all - I doubt anyone did a CFD study of the 3D models used in the anime series.

No there would not be much benefit to change the incidence angle of the main wings in flight. Dihedral plays mainly a role in passive lateral stability, and there are quite a few fighter planes around that don't have passive stability anyway - for instance the F-16 which has an aerodynamically unstable airframe in pitch, which is made controllable for a human through active stabilisation by the fly-by-wire computers.
As usual in aircraft design manufacturing, construction plays as large a role as aerodynamics do. And to make a wing that can make a flapping motion does not create any manoeuvrability benefit that I can think of, but does create large structural challenges. The main wings need to absorb the manoeuvrability loads of up to 9g, and it would be great if the projected wing area is not reduced by pointing the wing tips up.

Would there be a benefit to vertically adjustable main wings?

Not really, no.

Does "on takeoff and landing" count? Because the F-8 Crusader's variable-incidence-angle wings won the designers a Collier Trophy...

we are not talking pitch , the airplane can adjust the plane on the roll axis of rotation

The F-8s variable incidence wing really only changed the angle of the fuselage, ( the wing pretty much stayed at the same angle to the ground on approach). Lowering the fuselage angle allowed the pilot to see the runway, (the boat), much better.

@NajLitarvan Sorry, I do not understand what that means. Is it like in @
UnrecognizedFallingObject's comment? If so, ti requires a totally different answer and I will update it.

we not talking the wings rotating around the pitch axis(front to back), but roll axis (rotating from left to right, the same axis an aileron roll maneuver is executed.


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how are the properties of canards affected when they are angled compared to normal canards?
So i recently saw an image of the natf-23 a modified(more like completely redesigned) yf-23 and one of the interesting things it had were canards that were angled. What would the benefits and drawback be?
natf-23 model

if we go into fictional aircraft i also noticed it on the maneuvrability focused mode of the x-02 wyvern from the ace combat series.

Angling them up or down will wreak havoc with directional stability. This is plain silly.

Given that this is your third very similar question about fictional designs, I would vote for Peter's comment as an answer

When you have horizontal stabilizing surfaces with dihedral/anhedral, there is an effect on yaw stability from the effective vertical profile created by the angled surface. That vertical profile is as if you put a vertical fin there of equivalent area on the fuselage at that location.
The effective vertical surface area moves the vertical Neutral Point, the aerodynamic centre of the overall body, forward, reducing yaw stability (weathervaning tendency in yaw). This problem requires extra fin area on seaplanes when put on floats, which do the same thing to the vertical NP.
If the effective surface area is enough to move the NP to the C of G, the plane will just want point in any direction while flying (neutrally stable), and if forward of the C of G, it will want to switch ends like a dart you threw with the pointy bit toward you (in the plane's case, unstable in yaw, and once it's flipped around to point backwards, it will now be stable in yaw, going backwards).
If a designer wanted to make the plane neutrally or negatively stable in yaw on purpose, with active computer generated stability, rather than angle the canard surface, just making the existing vertical tails smaller would achieve the same end, and save a bunch of weight.
On the other hand, in theory, you could have those surfaces with variable anhedral/dihedral, and  get a variable natural yaw stability effect, high stability when flat, less stability when angled, if that was an objective (you will also have to deal with a stability side effect in the pitch axis for the same reason, because it will also shift the pitch Neutral Point fore/aft to some degree, to the extent that the surfaces' horizontal footprint is increase/decreased by the change in angle).
Nobody appears to have found a reason or justification to implement such a feature on a real fighter so far.
Anyway, it's important not to assume any consideration was given to how airplanes fly on the part of game developers, when you look at these fantasy configurations.

In addition, I wonder if there will be any additional fancy vortices stuff going on which has influence (good or bad) on the wing aerodynamics

That, plus rolling moments created during side slip, and I'm sure other effects that don't actually show up until you try out the real thing, that the computer models may or may not predict.

the funny thing is that the natf-23 was an actual proposed design irl at one point


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How much can a plane benefit from another planes wake?
Could a smaller plane fly in a V formation behind a much larger plane to reduce fuel consumption significantly more than a plane its own size? At what point does flying in a larger planes wake become to turbulent or increased fuel efficiency stops?

Related: How do planes benefit from flying in formation?

Dup of this post i think https://aviation.stackexchange.com/questions/16821/how-do-planes-benefit-from-flying-in-formation

@JohnK Not a duplicate but it is highly related and added it to the body thanks.

The image you posted is of a fighter jet lining up behind a tanker aircraft for refueling. The fighter would want to AVOID the wake turbulence, since it is extremely dangerous to fly through.

No, airplane wakes do not work like this. The streamlined body has very little wake, it is not like a cube shaped truck. The main wake on an airplane is found at the wingtips in the form of a strong vortex like horizontal tornado, aided somewhat by the down wash of the mid wingspan.
The tip vortex from a large plane will make a small plane spin like a windmill. A german bombardier challanger(mid size business jet) flew under an A380 at cruise altitude a few years ago and was turned over so hard the plane had to be sold for scrap, broke a one passanger's ribs, recorded near 4G force, lost 8000 ft of altitude before recovering control.

And then there are many cases where pilots weren't so lucky as to be able to regain control of the airplane before impact with the ground. XC-VMC comes to mind as a high-profile example.


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How far can the sound of a sonic boom travel?
Are there conditions or distances from the plane to cloud to the observer in which a sonic boom from a plane is muted?

Welcome to aviation.SE! I removed the comments about lightning from your question, because they seem to have nothing to do with sonic booms from aircraft. If I got it wrong, you can edit the question again or roll back my changes. You might also like to look at the questions we already have on sonic booms to see if they help.

My theory is the clouds containing water create a denser surface in which sound echoes or reflects off. Heat lightning or cloud lightning occurs higher off the cloud and above the base of the cloud. With the water being denser below the lightning or sonic boom sound is reflected up where ground lightning is acoustically contained between the cloud base and ground. I believe planes traveling at the right distance from the tops of clouds can reflect more sonic boom away from the ground.


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What plane has the widest range in speed in which it can operate?
What propellor plane, other than planes with vertical takeoff capability, has the largest gap in speed between its slowest stall speed and its maximum airspeed? I am looking for a plane with a short take-off but also can go fast if needed or slow and fuel-efficient.

The SR-71 lands at 155 knots, and has a top speed of +1900 knots, giving a speed range of about 1750 knots.

@ymb1 Thank you for correcting that. I will revise the question.

@Xaddell Range is the difference between v.max and v.min, so it's just the fastest plane question. Are you sure you want range and not ratio (v.max / v.min, division)?

If you're looking for ratio, I think one of the answers here will help: https://aviation.stackexchange.com/questions/65780/would-a-two-seat-light-fixed-wing-aircaft-with-a-landing-speed-of-20-knots-and-a

@ymb1 yes that question is spot on.

Does this answer your question? Would a two-seat light fixed-wing aircraft with a landing speed of 20 knots and a top speed of 180 knots be technically possible?


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What helicopter has the most rotor blades?
I have seen 2 rotor blades up to many and wonder when does having more rotor blades become less efficient? What helicopter has the most rotor blades?

Related: Why don't helicopters prefer shorter rotors with more blades?
What is the definining line between a helicopter and a drone a person can ride in?

If the blades are longer it has to spin slower to keep the tips from going supersonic. They are also heavier and harder to control

@RonBeyer do you think redundancy of having more blades could allow the helicopter to fly with damaged blades?

Unlikely. Chipped/mildly damaged blades can still fly, but once they fail structurally the entire rotor is unbalanced an no amount of extra blades is going to make it stabilize. It will shake itself apart as soon as it becomes unbalanced enough.

when does having more rotor blades become less efficient? -- any number more than one. The problem is if you need more lift then you need to use longer blades but if you cannot increase blade length due to structural issues and/or storage issues then you have no choice but to sacrifice efficiency for more lifting power. A single blade is ALWAYS the most efficient number of blades.

Is this blades per hub, or blades per helicopter? Tandem rotor (Chinook et al) or Coaxial rotor helicopters have several hubs, each with the same number/size of blades.

@CSM I think so, I could not find the picture a second time for the reference.

How are we defining "helicopter" here?  Your standard remote-control camera-drone toy creates lift in roughly the same way as a helicopter, using 4 rotors at the corners.  Each rotor can have 2-6 blades or so, giving you a total of potentially 24 blades.  I realize this is stretching the definition a bit, just wondering where you draw the line, since someone already brought up the Chinook 2-rotor varieties...

@DarrelHoffman As far as I'm aware, the are no passenger-carrying quad-rotors. Restricting this question to rotorcraft that can carry people (if only their pilot) is, I think, reasonable.

I wonder if there is an example of a helicopter carrying a load of other helicopter blades for some reason ... like a stack of them. Might win the contest...

@CSM The other question linked from this one (by the same person) shows a picture of exactly almost that: a passenger-carrying tri-rotor.  I don't know if that's just a prototype or not actually functional or what, but there appears to be at least one... (Edit: looks like there's only 3 rotors on further inspection)

(Reminding school lessions) Another interest for more blades is limiting low-frequency vibrations, as the resonance frequencies get higher when more blades are installed. This greatly improves the flying experience for humans, in terms of comfort and flight safety, as low frequency impedds vision among other things.

@DarrelHoffman has mentioned drones and then admitted they may not count as they do not carry passengers. But that leads to the question of why don't we build helicopters that way. Multiple rotors clearly have may advantages (or drones would not be built that way) and they are a pretty obvious way of having unlimited power without excessive blade length, blade-per-rotor number or blade speed. The only reason I can imagine is the difficulty of transmitting power from the single engine. Once electrical transmission, or even batteries become viable, I assume all helicopters will look like drones.

Funny how it works the same way for power-generating rotors: Wind powerplants are also most efficient with very small number of blades, in practice basically only 2 or 3 are used nowadays.

Special mention for Raúl Pateras Pescara de Castelluccio (good article) who was fond of lots and lots of rotor blades, settling on sixteen for most of his designs, although his Model 3 had twenty.
Pescara's helicopters may look a little comical (and dangerous!) but they are an important part of early helicopter evolution, pioneers in the first fully controlled helicopter flights, as well as the first use of the what would be recognised as modern helicopter controls.
Here's a video of Pescara testing one of his helicopters in 1922 wearing a Homburg hat with about 5mm of clearance to his head. He had many setbacks, but persevered. It's visionary, brave guys like him who make advances in aviation.
Plus, loads of blades.
Here is his Model 3 from 1929:
(source1)

Wow. Counter-rotating biplane rotors. What could possibly go wrong?

@TimLymington I have expounded the answer so it's hopefully less comment-like.

Mr Pescara did at least have the sense to fit a head guard to his machine. Very stylish guy, flight testing with that hat!

The most blades I've seen are 8 on the Mi-26.

Source
But the highest theoretical lifting efficiency is achieved with the fewest blades and experiments have been done with single blade rotors (with a counterweight - there were vibration problems that couldn't be resolved).
So in practical terms, the most lift for the least power is achieved with a 2 blade rotor, but the need to absorb more power with a limited desirable disc diameter forces you to add more blades (or you may want a smaller rotor with lots of blades for maneuverability or other reasons, but it's sub-optimal from a horsepower efficiency standpoint).
Put another way, a helicopter like the Mi-26 could probably achieve a fair bit more lifting power for its installed horsepower with a 2 blade rotor, but each blade would seemingly extend off into the next township.

It's interesting to note that the tail rotor of Mi-26 also has large number of blades (5).

@John K Could you maybe please provide some sources for your statements. Because for example the Ec145 helicopter recently got a new main rotor with five instead of 4 baldes and now it can carry more load with the same engine power     https://www.verticalmag.com/news/airbus-helicopters-launches-5-bladed-h145-upgrade/

Added a link to another ASE post where @Peter Kämpf responds with much greater authority than I on the theory.

"The blades incorporate the latest airfoil design to produce more thrust than previous blades at the same power. While five blades have more drag than four blades in a rotor, Humpert said improvements to the aerodynamics of the rotor cuff of the blade had actually allowed the new H145’s blades to provide more lift. “This is the aerodynamic evolution which is technically behind it,” he said."  Adding the extra blade in itself wasn't the benefit. It was a package of optimizations; blade airfoil, root design, etc.  Fewer blades are still more efficient than more blades, all else being equal.

Than riddle me this why didn't they just take 4 optimised new blades instead of 5 if 4 according to your claims is more efficient. Yes you run into problems with wake with more blades, but with less blades you also need to increase the size of your baldes which brings other porblems. so it isn't just black an white there is a sweetspot in the middle for the blade number

Probably because they discovered that the efficiency improvements resulted in more power available than could be absorbed by 4 blades, necessitating adding a 5th.

If, theoretically, more thrust is achieved with the fewest blades - would that mean that, theoretically, a single blade prop is best for an airplane?  And, even, theoretically, a single blade turbine is best in a jet engine?  (a very wide jet engine, but this is theory!!)

Well jets is getting a bit out there but yes.  It's well known in the airplane business that I'll get more static thrust out of a longer 2 blade prop than a shorter 3 blade prop with the same overall blade area, although the difference is small enough to make it worthwhile sometimes to use the 3 blade for the lower noise levels and better tip clearance.

Luck is approximately the last thing I want to be keeping a 737 in the air...

It's important to remember that the rotor blades of a helicopter are basically wings, and thus there is a finite amount of lift you can extract from them before they stall. More blades = more friction and lower efficiency, but also reduced loading on each individual blade (and thus reduced chance of stall).

@DavidRicherby indeed, I'd rather have bloodymindedness :)

If the most lift for the least power is achieved with a 2 blade rotor, what was Pescara doing? Why build it with (presumably not) the most power, for the least lift?

He was trying to get the required blade area in a limited span with a rotor made from sticks spinning on a big pipe.  You need a certain area, and you can increase span or chord or both, or add more blades or all of the above.  Most helicopters use multiblade rotors to get the required blade area while limiting rotor diameter, because the benefits outweigh the theoretical penalty.  Doesn't change the theoretical ideal of fewest blades for the required area.

@JohnK I was thinking about the space between the blades. On smaller helicopters with less blades the distance between the blades tips are equal then the larger helicopter with more blades allowing having the same wake?

There's more to it than just the wake effects and spacing.  On a two blade you have only two roots and tips with related losses.  On a 5 blade rotor of the same total blade area say, you now have 5 roots and tips with related losses for the same lifting area. Put another way, a 2 blader is a little bit closer to the theoretical infinite span wing with no tip losses than a 5 blader.   That being said, the benefits of a multi-blade rotor in many other ways more than make up for the difference on complex machines.  For simplicity and the most lift on the least power however,2 blade is still best.

authors: Bob Jarvis - Слава Україні


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What is the definining line between a helicopter and a drone a person can ride in?
When I say drone in this context I don't mean an unmanned aircraft. I'm not sure what the terminology of a drone that has been adapted to carry a person? Is this picture below considered a helicopter or what? What if it ran on petroleum?

If not, then how is the separation between a VTOL aircraft and helicopter defined?
Would a Chinook that can fly itself be a drone?

Can that thing autogyro?

If it can fly itself it is an AAV, drones are dumb by definition.

I think the word you're looking for is "quadcopter". The aircraft in the picture you posted is a quadcopter. That aircraft has nothing to do with drones whatsoever, aside from the fact that it's a quadcopter, and many drones are quadcopters.

I'm assuming, by the way, that the aircraft in your picture is controlled by a pilot sitting inside and cannot be controlled remotely or autonomously.

@TannerSwett some models could fly remotely and have a return to home feature.

@TannerSwett: That's actually a hexacopter or a contra-rotating tricopter

@slebetman, No, it's an octacopter or a contra-rotating quadcopter. Look at the shadows.

There's a whole lot of questions in your question. Which is your primary question?

@Spook Shadows aside, it would seem to be weirdly balanced if at all with only three arms.

@Muze - Very good question! I hope this one and this one are enough to get you out of question ban jail. Good luck!

@SteveV. But if I had started with the https://aviation.stackexchange.com/questions/62591/which-helicopter-can-auto-rotate-the-farthest I would have been banned for another 6 months.

There is no automatic separation between the two, it is possible for an aircraft to be both a drone and a helicopter. Drone only refers to how the vehicle is piloted, and helicopter only refers to how it flies. (Battery vs petroleum vs any other power source is a third, separate modifier, too.)
The vehicle in your picture is an unmanned aircraft, a drone, a helicopter, a quadcoptor, and a VTOL vehicle all at once.
(Note: Unmanned means not having any crew on board. You can still have passengers, they don't count.)

It does raise the interesting question where the distinction between a "passenger" selecting a destination on a map and a "pilot" putting a waypoint into a full autopilot is.  Ability to take manual control is of course a training and likely regulatory difference.  But that will never be flown without some computer assistance, such as a rate-damping loop managing the throttles (ie, what RC quad people call "acro mode")

It is also an R/C aircraft...

A helicopter is an aircraft in which thrust and lift are provided by rotors.
A drone is an unmanned, self-piloted or remotely-controlled aircraft, which can use rotors to provide thrust and lift, but can also use other means such as propellers or turbines.
To put it in simpler terms, a helicopter can only be a helicopter, regardless of whether it has a human inside or not. A drone can be any kind of aircraft.

I know some people like to clarify it even further and insist that a "drone" is a self-piloting unmanned craft, rather than one remote-controlled by a human on the ground.  There are people (mostly military) who get really offended when you refer to a human-controlled unmanned craft as a "drone".  (Much like the infamous boat vs. ship debate - do NOT get that wrong in front of Navy people...)

A drone with a ground operator pretty much flies itself. The operator is there mostly to command changes to mission profile and make the kinds of life and death decisions the computer can't make (and even that will eventually be done away with with the use of machine learning). If I am not mistaken, all the autonomous and ground-operated drones have the capability to fly home on their own, either when commanded or if they lose contact with home base.

"do NOT get that wrong in front of Navy people..."  It depends on the Navy people you are in front of.  I was a Naval Aviator and we called everything that floated a boat!  :)  Surface types don't share the same sense of humor though.

A drone is generally useless unless it flies some sort of payload. Just because the payload is a sack of water and meat doesn't mean it's not a drone. :)

@MichaelHall I was going to say the same thing, in the Corps we called them boats and the squids... er... sailors never got offended. :)

@MichaelHall I'm surprised you didn't just lump them all into 'targets' to really annoy the surface sailors.

The term drone comes from the original UAVs that flew preset paths "dumbly" (like worker bees AKA drones): https://www.popsci.com/technology/article/2013-03/drone-any-other-name

I would just call it a VTOL aircraft.  Those kind of multi-rotor VTOLs do kind of blur the line between VTOL and helicopter, but a practical way to define them could be to separate them by a key capability; the ability to glide.
A helicopter has a rotary wing that is driven forward by a power source like an airplane, but can also change pitch and glide downhill, like an airplane.  A rotor is just two fuselage-less airplanes joined at the wingtips being driven in a circle by a twisting force applied at the wing tip instead of a propeller out at mid span.
A multi-rotor VTOL with small fixed pitch rotors can't glide, which is why you won't get me in one unless it has multiple levels of redundancy with NO single-points-of-failure or double-points-of-failure modes for that matter (like say being able to take damage to both rotors on one corner for example, and still maintain control).

"A rotor is just two fuselage-less airplanes" It can be more than two.

"fuselage-less airplanes" ... so wings?

Yeah... as in "rotary wing" aircraft.

Theoretically that design could operate with one corner out.  First all the sets are counter-rotating, so there's no torque issue.  And it's hybrid electric, so it is just a controls problem to provide reverse thrust from the opposite corner if needed for balance.  However they seem to have hedged their bets with a whole aircraft ballistic parachute.

Yeah I've heard that too.  Ballistic parachutes are no good below a certain height, so you still end up with a critical risk zone.  I have no doubt however that at some point they will be considered to be safe people movers.

When I say drone in this context I don't mean an unmanned aircraft.

Drones are unmanned aircraft which can receive flight path control (rather than controls directly affecting the control surfaces/rotors). Method of generating lift, engine technology and size are not a factor - after WW1 some biplanes became drones and various air forces have converting more modern end-of-life fighters to drones, either for attack or for target practice.

I'm not sure what the terminology of a drone that has been adapted to carry a person?

Passenger carrying unmanned aerial vehicle or 'passenger drone'

Is this picture below considered a helicopter or what?

What. I'd call it a rotorcraft (UK terminology used, e.g. in CAA larger UAS guide), or more specifically a large octocopter.
I tend not to call things helicopters if they don't have helicopter controls (cyclic and collective driving a swashplate), but I can't find an authority on whether that is what makes a helicopter. Electric multi-rotor craft use differential rotor speed to manoeuvre instead.

What if it ran on petroleum?

I suspect it would fall out of the sky - it would be hard to get the control responsiveness from eight small petrol engines or one large one and an eight way variable transmission that would allow it to be stable. You could change it from an octocopter to an eight rotor helicopter by adding swashplates, but with the other changes to make that happen it would be a different thing.

"Drones are unmanned aircraft which can receive flight path control" False. As demonstrated by your own link, the terminology originated before such capability even existed, specifically at the point where they were turning full scale piloted aircraft into a larger version of later hobby RC planes

@ChrisStratton yes, historically 'drone' was applied before automated wayfinding. Also 'computer' used to be the word for a human who did arithmetic, but if I someone asked me what a computer is I wouldn't give that as the definition.

The meaning of "drone" has not changed, it is still fully inclusive of the type of remote piloted operation you are trying to exclude.

@ChrisStratton in my own lifetime it has gone from being exclusively military fixed wing to include quadcopters and the like. The distinction between a small drone and a RC plane is the wayfinding. The larger modern drones also all have wayfinding.

The majority of quadcopters are remotely piloted.  Only higher end systems have mission-level autopilot that follows GPS waypoints or terrain match cameras.  But all of the above are within the meaning of "drone" - making your attempt to exclude the remotely piloted aircraft which coined the term, and those that still work that way, silly.


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Why is 121.5 called "guard"?
What is a "guarded" frequency and how does it differ from a frequency that's unguarded?

It is called guard because everybody is supposed to listen/guard the frequency just in case someone has a problem.
Guard definitions:

a state of caution, vigilance, or preparedness against adverse
  circumstances
watch over in order to protect or control.

The AIM 6-3-1(h)(1) says (emphasis mine):

121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by direction finding stations and
  some military and civil aircraft. 243.0 MHz is guarded by military
  aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military
  towers, most civil towers, FSSs, and radar facilities. Normally ARTCC
  emergency frequency capability does not extend to radar coverage
  limits. If an ARTCC does not respond when called on 121.5 MHz or 243.0
  MHz, call the nearest tower or FSS.

121.5 is also guarded by that guy who never hesitates to radio "On guard!" when someone accidentally transmits on guard.

en guarde mom ami!

@rbp: En garde mom! (but if you aren't talking to your mom, then it's: En garde mon ami !).

" India Golf niner-niner transmitting in the blind guard. Disengage! Repeat, disengage! "

@casey: When professional pilots are anything but

Two reasons:
First, and most relevant today, all facilities are supposed to "guard" (monitor) the frequency if able. Most ATC facilities and Flight Service stations monitor 121.5MHz, as do many airliners or aircraft with two radios.  Lnafziger already gave you the relevant AIM paragraphs that talk about this.
Second, "guard frequencies" in general radio terminology are frequencies that have extra protection in the band.
This is done for two reasons: to prevent use of the guard channel from interfering with other channels, and/or to ensure that the guard channel is always clear and that adjacent channels won't "bleed over" and render it unusable. This mattered more in the days of tube radios with poor frequency alignment & confinement - modern radios bleed much less, but some frequencies are still afforded extra band protection.
121.5MHz in the USA is guarded in both respects - monitored as Lnafziger describes, and afforded extra band protection (50KHz on either side, 121.425 - 121.475 MHz & 121.525 - 121.575 are unusable per AC 90-50).

As long as you are talking about radio theory, a guard frequency also frequently can not be over-ridden.  Ie, if ATC is transmitting on a secondary frequency, someone transmitting on the guard frequency can still be heard.

@Lnafziger Yup - my understanding is most ATC facilities are set up so 121.5 is always heard by someone - usually on a facility speaker and/or monitored in headsets either by everyone or at specific sectors. (Facilities that don't monitor 121.5 at all usually have a notice to that effect somewhere - like "Guard frequency 121.5 not available at tower" in the AFD remarks)

I wonder whether it comes from the French, where the verb "guarder" means "to keep" or "to reserve" or "to keep in reserve". A "guarded" frequency would then mean "a reserved frequency" which makes perfect sense.
Note that Mayday comes from the French.

Google says that the word guard (not just the aviation term) comes from Old French garde and garder.

I was surprised to find French in those other 'Mayday'-related words in Wikipedia: "pan pan", "sécurité", "silonce", etc. The term "guard band" is also used in radio (meaning, unused to prevent interference).

France was pretty heavily involved in early aviation, so that's probably got something to do with it!

Fuselage empennage aileron etc

From my days as a Navy communications officer, if you guarded a frequency such as 121.5MHZ you also had a transmitter set for that frequency as well as a receiver. If you only monitored a frequency, you only had a receiver tuned to it. We at least monitored both 121.5 and 243.0 any time we put out to sea.

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